From EP-A 100 118 it is known that olefins can be converted with hydrogen peroxide to epoxides when a titanium containing zeolite is used as catalyst. The reaction is preferably carried out in the presence of a polar solvent which dissolves both the olefin and the hydrogen peroxide in the reaction mixture. Methanol is preferably used as solvent.
Nitrogen containing compounds are frequently introduced into the process as components of the starting materials, as additives to the epoxidation reaction or as additives or auxiliaries during the work-up of the reaction mixture.
Nitrogen containing compounds may be introduced with the hydrogen peroxide used. Commercial hydrogen peroxide solutions often contain nitrogen containing compounds as peroxide stabilizers or as corrosion inhibitors. One example is Aminotrimethylenephosphonic acid used as a stabilizer.
Nitrogen containing compounds can be added to the epoxidation reaction to improve the propylene oxide selectivity. A large range of nitrogen containing compounds has been described for this purpose in available publications; for example: Ammonia, ammonium salts and nitrates in EP-A 230 949 and EP-A 712 852, organic nitrogen bases such as amines in EP-A 1 072 599 and WO 00/17178, amine oxides in WO 00/17178, carboxylic acid amides in EP-A 940 393 and WO 01/92242 and nitriles in WO 01/68623. In a similar fashion, nitrogen containing chelating agents have been added to the epoxidation mixture to suppress the decomposition of hydrogen peroxide as described in EP-A 757 045. All of these listed publications are relied on and incorporated herein for their disclosures of nitrogen compounds.
An example for introducing a nitrogen containing compound during the work-up of the epoxidation mixture is disclosed in WO 99/14208, incorporated herein by reference, where nitrobenzene is described as the solvent for the extraction of the olefin oxide from the reaction mixture.
When the epoxidation process is performed on an industrial scale the polar solvent used in the epoxidation reaction has to be recovered in the work-up of the reaction mixture and recycled to the epoxidation reaction for economic reasons. Processes for the recovery of the solvent have been described in WO 99/07690, WO 99/08985, WO 99/11639, WO 02/02544, WO 02/02545, WO 02/14298, WO 02/00635, WO 99/14208, WO 99/23052, WO 02/00634, U.S. Pat. No. 5,599,955, WO 01/57009 and WO 01/57010. When the solvent is recycled, impurities contained in the recycled solvent may accumulate in the recycle up to undesirable levels. This problem has been addressed for the impurity acetaldehyde in WO 99/07690 and WO 99/08985 and for methyl formate in WO 02/02544 and WO 02/02545.
The recycled solvent may also contain impurities poisoning the epoxidation catalyst which leads to lower catalytic activity and a more rapid deactivation of the epoxidation catalyst when the recycled solvent is used in the epoxidation reaction. The inventors have observed such a catalyst poisoning when one or several nitrogen containing compounds were introduced at some stage of the oxidation process. The known methods for recovering and recycling the solvent have not addressed the problem of catalysts poisoning and offer no solution to this problem.
Therefore it is an object of the present invention to improve a process for the catalytic epoxidation of olefins in the presence of a titanium containing zeolite catalyst wherein one or more nitrogen containing compounds are introduced at some stage of the process to achieve an improved activity of the epoxidation catalyst when solvent is recovered and recycled to the epoxidation stage.